1. Field of the Invention
The present invention relates to a method for controlling a speed of a single phase induction motor in which the control of the speed is performed by varying a frequency of a control signal.
2. Description of the Prior Art
Generally, rotation speeds of conventional single phase induction motors are determined according to the frequency of a drive voltage which is supplied from a commercial power source. For this reason, the conventional single phase induction motors are designed in such a manner that the rotation speeds thereof are controlled based on a commercial frequency of 60 or 50 HZ.
FIG. 3 is a schematic block diagram of a conventional apparatus for controlling the speed of such a single phase induction motor, FIG. 4A is a timing diagram of an output signal from an encoder in the apparatus in FIG. 3 and a drive signal to the single phase induction motor when the rotation speed of the motor is low, and FIG. 4B is a timing diagram of the output signal from the encoder in the apparatus in FIG. 3 and the drive signal to the single phase induction motor when the rotation speed of the motor is high. Here, the single phase induction motor is designated as the reference numeral 34.
In operation, when the variable speed motor 34 is is operating at a low speed, an interrupt signal is applied to a central processing unit (CPU) 33 at a rising edge of an output signal from an encoder 31 every one cycle of that signal as shown in FIG. 4A. In response to the interrupt signal, the CPU 33 performs an arithmetic operation for a drive signal DS for a time period of about 0.1 msec and then outputs the resultant drive signal DS to a motor driving circuit 32, thereby causing the driving circuit 32 to control the rotation of the motor 34.
In the case where the motor 34 is operating at a high speed, the interrupt signal is applied to the CPU 33 at the rising edge of the output signal from the encoder 31 every two cycles of that signal as shown in FIG. 4B. Similarly, upon receiving the interrupt signal, the CPU 33 performs the arithmetic operation for the drive signal DS for a time period of about 0.1 msec and then outputs the resultant drive signal DS to the motor driving circuit 32. In response to the drive signal DS outputted from the CPU 33, the driving circuit 32 controls the rotation of the motor 34.
In other words, the arithmetic operation of the CPU 33 is skipped over one of the two cycles of the output signal from the encoder 31. This makes the output time of the drive signal DS from the CPU 33 short. The short output time of the drive signal DS results in an increase in a duty ratio of an ON interval of the drive signal DS. As a result, the rotation of the motor 34 and, thus, the speed thereof can be controlled even at a high speed.
As mentioned above, in order to control the speed of the motor 34, a time period of about 0.1 msec is required for the arithmetic operation of the CPU 33 with respect to the drive signal DS, and the interrupt signal is applied to the CPU 33 at the rising edge of the output signal from the encoder 31 every one cycle or two cycles of that signal. For this reason, the CPU 33 must stop the output of the drive signal DS to the driving circuit 32 for the arithmetic operation regarding the drive signal DS. Then, upon completion of the arithmetic operation, the CPU 33 outputs the drive signal as a result of the arithmetic operation to the driving circuit 32.
In this connection, the output time of the drive signal DS from the CPU 33 becomes abnormally short, particularly when the motor 34 is operating at a high speed, because the arithmetic operation of the CPU 33 is skipped over one of the two cycles of the output signal from the encoder 31 and requires a time period of about 0.1 msec. For this reason, it is difficult in practice to control the rotation of the motor 34 at a high speed.
On the other hand, in a conventional drive motor for an electric fan in which the rotation speed is controlled by three steps, low, middle and high, low, middle and high speed induction windings must individually be provided in the drive motor. The low, middle and high speed induction windings are connected respectively to different induction windings through a switch for the control of the rotation speed of the drive motor. In this connection, the conventional electric fan drive motor has a disadvantage in that it is complex in construction since the various windings are provided therein. Also, the speed cannot be varied successively.